Because the accused is still a citizen. How would you feel if a DA decided to charge you against his better judgment merely because the mob is demanding it and the DA risks losing reelection if he declines to prosecute you?
The DA's job is to prosecute a suspected criminal when he or she has sufficient evidence to present to the court that:
1) a crime took place and
2) the suspect did it.
And that determination should not be affected by political considerations.
In the case of Officer Van Dyke, the evidence that a crime took place and he's the one who committed it isn't just sufficient, it's overwhelming. The Prosecutor utterly failed to do her job. I for one am glad her thumb is no longer tipping the scales of justice in favor of cops who murder suspects.
But she did charge him. In fact, she overcharged him as there is no evidence what he did constitutes "1st degree murder" but is probably manslaughter. After all, this #BLM hero was menacing people with a knife and even attacked the responding cop car. The initial shots were perfectly justified, where Van Dyke went wrong is that he continued shooting after the threat was neutralized.
And why is the DA taken to task so much for taking too long to charge a police officer when there are many many more non-police shootings in Chicago than police shootings? There are more than 400 murders in Chicago every year. Shouldn't that be a priority and not the death of some crazy knife wielding maniac with an extensive juvie record?
And as far as Tamir Rice, I see no reason the DA did not follow his best professional judgment. In the older thread on the case, recently revived, it says that two independent experts were of the opinion that the shooting, as tragic as it was, did not constitute a crime by the officers. Should the DA have charged them anyway just to appease the mob?